Electronic module assembly with patterned adhesive array

ABSTRACT

An improved electronic module assembly and method of fabrication is disclosed. A patterned array of adhesive is deposited on a laminate, to which a chip is attached. Each region of adhesive is referred to as a lid tie. A lid is placed on the laminate, and is in contact with the lid ties. The lid ties serve to add stability to the laminate and reduce flexing during thermal processing and mechanical stress.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of commonly-owned, copending U.S. patentapplication Ser. No. 13/939,606 entitled ELECTRONIC MODULE ASSEMBLY WITHPATTERNED ADHESIVE ARRAY and filed on Jul. 11, 2013.

FIELD OF THE INVENTION

The present invention relates generally to the fabrication of electronicdevices, and more particularly, to an electronic module assembly with apatterned adhesive array and method of fabrication.

BACKGROUND OF THE INVENTION

Electronic modules which use a cover or lid need to be assembled in amatter that provides mechanical protection while addressing thedimensional, mechanical, thermal and in some cases electricalrequirements of the assembly. During a traditional assembly of die ontoorganic laminates and subsequent adhesive lid attach to the laminate anddie, significant stresses can be developed due to respective materialthermal expansion mismatches that cause the laminate to warp. Thiscomplicates the placement of bottom side solder balls (where ball gridarray (BGA) packages attach to cards) if the warpage is severe, sincethe balls will not sit on a common plane to reliably join to a planarcard input/output (IO) array surface with solder. Similar planarityissues are encountered for modules which are joined to, for instanceLand Grid Array (LGA) connectors which use compliant contacts such assprings to create electrical paths between the laminate surface and asurface such as a card, board or other electrical interface. Thecontacts may not all be fully engaged or forcefully mated to bereliable. This problem, referred to as a coplanarity issue, is magnifiedif the laminate is of low rigidity as is found with coreless laminates,as coreless laminates do not have the traditional thick glass fiberstiffening central core.

In addition to the coplanarity issue, a variety of other thermal andmechanical challenges exist in the fabrication of electronic modules.The warpage that results can have an adverse effect on the thermalinterface material (TIM) that resides between the die and the undersideof the lid. This TIM provides a physical thermal path for heat generatedin the die to flow to the lid and from there be further removed by usingheatsinks, coldplates, etc. to prevent overheating of the package andmaintain electrical performance of the module. Therefore, it isdesirable to have an improved electronic module and methods offabrication that mitigates the aforementioned problems.

SUMMARY OF THE INVENTION

One aspect of the present invention provides an electronic moduleassembly comprising: a laminate; a plurality of contacts disposed on abottom side of the laminate; a semiconductor chip disposed on a top sideof the laminate; a seal band disposed on the laminate, forming aperimeter around the chip; a plurality of stabilizing lid ties disposedon the laminate in a region within the perimeter; and a lid disposed onthe chip and the laminate, wherein the lid is in contact with the sealband and the plurality of stabilizing lid ties.

Another aspect of the present invention provides an electronic moduleassembly comprising: a coreless laminate; a plurality of contactsdisposed on a bottom side of the laminate; a semiconductor chip disposedon a top side of the laminate; a thermal interface material layerdisposed on the chip; a seal band disposed on the laminate, forming aperimeter around the chip; a plurality of stabilizing lid ties disposedon the laminate in a region within the perimeter; and a copper liddisposed on the chip and the laminate, wherein the lid is in contactwith the seal band, the thermal interface material layer, and theplurality of stabilizing lid ties.

Another aspect of the present invention provides an electronic moduleassembly comprising: a laminate; a plurality of contacts disposed on abottom side of the laminate; a semiconductor chip disposed on a top sideof the laminate; a seal band comprised of a plurality of seal band lidties disposed on the laminate, forming a perimeter around the chip; alid disposed on the chip and the laminate, wherein the lid is in contactwith the plurality of seal band lid ties.

Another aspect of the present invention provides a method forfabricating an electronic module assembly, comprising: placing a chip ona laminate; forming a seal band on the laminate; depositing a patternedarray of stabilizing lid ties to the laminate; applying a lid on to theseal band; and curing the electronic module assembly.

BRIEF DESCRIPTION OF THE DRAWINGS

The structure, operation, and advantages of the present invention willbecome further apparent upon consideration of the following descriptiontaken in conjunction with the accompanying figures (FIGs.). The figuresare intended to be illustrative, not limiting.

Certain elements in some of the figures may be omitted, or illustratednot-to-scale, for illustrative clarity. The cross-sectional views may bein the form of “slices”, or “near-sighted” cross-sectional views,omitting certain background lines which would otherwise be visible in a“true” cross-sectional view, for illustrative clarity.

Often, similar elements may be referred to by similar numbers in variousfigures (FIGs) of the drawing, in which case typically the last twosignificant digits may be the same, the most significant digit being thenumber of the drawing figure (FIG). Furthermore, for clarity, somereference numbers may be omitted in certain drawings.

FIG. 1A is a side cutaway view of an electronic module assembly inaccordance with embodiments of the present invention.

FIG. 1B is a side cutaway view of an electronic module assembly inaccordance with additional embodiments of the present invention.

FIG. 2 is a top-down view of an electronic module assembly in accordancewith embodiments of the present invention, indicating the seal band andlid ties.

FIG. 3A is a top-down view of an electronic module assembly inaccordance with embodiments of the present invention, indicatingelectrical components.

FIG. 3B is a top-down view of an electronic module assembly inaccordance with embodiments of the present invention, indicating a sealband comprised of lid ties.

FIG. 4 is a system for fabricating an electronic module assembly inaccordance with embodiments of the present invention.

FIG. 5 is a flowchart indicating process steps for embodiments of thepresent invention.

DETAILED DESCRIPTION

FIG. 1A is a side cutaway view of an electronic module assembly 100 inaccordance with embodiments of the present invention. Assembly 100comprises laminate 102. Laminate 102 comprises multiple electricalcontacts and wiring routes, as is known in the art (not shown). Inembodiments, laminate 102 may comprise a coreless laminate. Corelesslaminates do not have the traditional thick glass fiber stiffeningcentral core. This allows greater wiring flexibility with corelesslaminates, as wires and vias can be placed closer together than withcore (e.g., Thincore) laminates. However, coreless laminates provideless mechanical rigidity than a core laminate, which has createdchallenges in the fabrication and subsequent assembly processes.Embodiments of the present invention may also be utilized with corelaminates. Hence, in some embodiments, laminate 102 may comprise a corelaminate. On the bottom side of laminate 102 is a plurality ofelectrical contacts 106, each of which is a ball comprising part of aBall Grid Array (BGA). Disposed on the top side of laminate 102 is a die(chip) 112. Chip 112 may include a logic chip, digital signal processor,microcontroller, microprocessor, graphics processor, or any othersemiconductor chip. Chip 112 comprises multiple solder bumps 114, whichmake electrical contact with the laminate 102. In embodiments, thesolder bumps 114 may comprise Controlled Collapse Chip Connection (C4)solder bumps. Disposed on chip 112 is a thermal interface material (TIM)layer 116. Note that the drawings are not to scale, and the TIM layer istypically much thinner than chip 112. The TIM layer serves to conductheat away from chip 112, and on to lid 104. Lid 104 may be comprised ofmetal. In some embodiments, lid 104 may be comprised of copper. In otherembodiments, lid 104 may be comprised of aluminum. Lid 104 is adhered tolaminate 102 by seal band 110. In embodiments, seal band 110 may becomprised of a silicone adhesive. In some embodiments, the siliconeadhesive may comprise an epoxy or an RTV (room temperature vulcanizing)silicone adhesive which is applied at room temperature (about 18 degreesCelsius to about 24 degrees Celsius). The region 117 of laminate 102under the seal band 110 is fairly well supported by lid 104. The region115 of laminate 102 under the chip 112 is fairly well supported by thechip 112. However, unsupported region 118 is prone to flexing andwarpage. To reduce the flexing and provide additional mechanicalstability, a patterned array of adhesive is applied. Each element of thepatterned array of adhesive is referred to as a “lid tie.” As shown inFIG. 1A, two stabilizing lid ties 108 are visible. The stabilizing lidties 108 may be comprised of the same material as seal band 110. In someembodiments, the material for the lid ties 108 and seal band 110 isEA6700 silicone adhesive from Dow Corning Corporation of Midland, Mich.

FIG. 1B is a side cutaway view of an electronic module assembly 150 inaccordance with additional embodiments of the present invention.Electronic module assembly 150 is similar to electronic module assembly100, except that on the bottom side of laminate 102 is a plurality ofcontacts 121, each of which is a landing pad, comprising a Land GridArray (LGA). The land grid array is a packaging technology with a gridof contacts on the underside of a package that uses pressure toestablish electrical connection between adjacent contacts. The LGAcontacts can be connected to a grid of contacts on the PCB. In somecases, the contact is made by using an LGA socket. In other cases, theLGA may be soldered directly onto a printed circuit board (PCB) on onesurface leaving the opposing surface contacts available for connectionto a package without the use of solder.

FIG. 2 is a top-down view of an electronic module assembly 200 inaccordance with embodiments of the present invention, without the lid,indicating the seal band and lid ties. As stated previously, similarelements may be referred to by similar numbers in various figures (FIGs)of the drawing, in which case typically the last two significant digitsmay be the same. For example, laminate 202 of FIG. 2 is similar tolaminate 102 of FIG. 1. The seal band 210 is disposed on the laminate,forming a perimeter around the chip 212. A thermal interface material(TIM) 222 is applied to the top of the chip 212. This will spread out tocover the entire top of the chip 212 once a lid is applied and forcedonto the die during the lid attach process.

A plurality of stabilizing lid ties 208 is disposed on the laminate. Inthis embodiment, a patterned array of lid ties on the laminate comprisesan outer ring 224 of lid ties and an inner ring 226 of lid ties. In someembodiments, between 26 and 30 lid ties are disposed on laminate 202. Insome embodiments, 28 lid ties are disposed on laminate 202. In someembodiments, 16 lid ties are in the outer ring 224, and 12 lid ties arein the inner ring 226. As the lid tie material has a non-zero cost,using more material than necessary can add up to a significant cost fordevices produced in high volumes. It is therefore desirable to use theminimal amount of lid ties that are needed in order to providesufficient stability. The stabilizing lid ties 208 may be round, asshown. In other embodiments, other geometries can be used, such aslines, or oval-shaped lid ties.

FIG. 3A is a top-down view of an electronic module assembly 300 inaccordance with embodiments of the present invention, indicatingelectrical components. The electrical components 328 may include, butare not limited to, capacitors, MEMs devices, resistors, diodes, andinductors. It is preferable to maintain a minimum component spacingdistance D1 between each stabilizing lid tie 308 and its nearestelectrical component 328. In some embodiments, the minimum componentspacing distance ranges from about 25 micrometers to about 700micrometers. The minimum component spacing distance reducescomplications during downstream processing. For example, if a lid tiewere too close to, or disposed on certain types of electricalcomponents, it could adversely impact solder reflow steps or solder joinreliability. Therefore, the position of the lid ties is a predeterminedpattern which allows room and positional flexibility for placement ofelectrical components, while still having a sufficient density per unitarea to provide additional support for the laminate. In someembodiments, the unit area 330 is ten square millimeters, and theaverage lid tie areal density is about 3 lid ties per 10 squaremillimeters. A distance between two lid ties, referred to as a lid tieinterspacing distance D2, is a parameter that affects the average lidtie density. In some embodiments, the average lid tie interspacingdistance ranges from about 2 millimeters to about 20 millimeters. Insome embodiments, the average lid tie interspacing distance ranges fromabout 6 millimeters to about 10 millimeters. These values and placementpositions are optimized for each design and can be verified usingthermomechanical models.

In some embodiments, a stabilizing lid tie may be used on a component,and may completely cover the component. In some cases the joiningtechnique for components allows a lid tie to contact a component, suchas for C4 attached decaps which are underfilled, conductive epoxy joins,higher T solder joins that do not re-melt during BGA attach, or for LGAmodules.

FIG. 3B is a top-down view of an electronic module assembly 350 inaccordance with embodiments of the present invention, indicating a sealband 317 comprised of a plurality of seal band lid ties, indicatedgenerally as reference 319. In some embodiments, the seal band lid ties319 may have the same size as the stabilizing lid ties 308. The sealband lid ties 319 may be made from the same material as stabilizing lidties 308, or in some embodiments, the seal band lid ties 319 may becomprised of a different material than stabilizing lid ties 308. Theseal band lid ties 319 may have an interspacing distance D3. In someembodiments, the seal band lid tie interspacing distance D3 may rangefrom about 0.7 millimeters to about 2.3 millimeters. The greater theinterspacing distance D3, the less material is used, and hence,additional cost is saved. However, there is a tradeoff between theinterspacing distance D3 and the adhesion of the lid. If too few sealband lid ties are used, the lid may not be sufficiently secured to thelaminate 302.

FIG. 4 is a system 400 for fabricating an electronic module assembly inaccordance with embodiments of the present invention. A componentplacement tool 440 is configured to place electrical components on thelaminate. The component placement tool may use vacuum to pick up partsfrom a dispensing mechanism such as a tape feeder, and place them on thelaminate at the appropriate location and orientation. An adhesivedispensing tool 442 is configured to deposit the seal band and lid ties.In embodiments, the adhesive dispensing tool 442 uses a roboticallycontrolled syringe to dispense the adhesive that makes up the seal bandand lid ties. In some embodiments, the adhesive dispensing tool 442dispenses lid ties that have a height of about 1.5 millimeters to about2 millimeters. The placement of the lid ties enables critical areas tobe reinforced more than other, less stressed regions. For example,additional lid ties may be placed near the corners of a chip foradditional reinforcement in those areas. Selective placement providesflexibility in avoiding regions where adhesives could create damage tocomponents (e.g. overcoating MEMS, or tunable circuitry). Inembodiments, the placement may be based on where electrical componentsmay exist. The locations of electrical components may be designated aslid tie prohibit or ‘keep out’ regions, where no lid tie can be placed.In embodiments, the adhesive dispensing tool may receive data from anelectronic design automation (EDA) system 443. The received data mayinclude lid tie prohibit region location information. The EDA system 443may comprise a processor 461 which is configured to access memory 463.Memory 463 may be a non-transitory memory containing instructions, whichwhen executed by processor 461, determine a location for each lid tie.Various parameters may be used in the determination of lid tielocations, including, but not limited to, a desired lid tie density perunit area, a minimum component spacing distance, and a desired lid tieinterspacing distance. The EDA system 443 may compute positionalinformation for each lid tie and transmit the positional information tothe adhesive dispensing tool 442 so that lid ties are dispensed in thedesired locations.

Patterned deposition of lid ties greatly reduces the quantity ofmaterial used to reduce overall costs of making assemblies (in somecases, the lid tie material may cost a few dollars per gram). Once thelid ties are in place, along with the thermal interface material andseal band, a lid application tool 444 applies a lid to the electronicmodule assembly. The lid makes contact with the lid ties, and compressesthe lid ties, causing the lid ties to spread out to a compresseddiameter. Once spread out, the lid ties may have a compressed diameterranging from about 1.8 millimeters to about 2.3 millimeters. The lidapplication tool 444 may utilize a vacuum to pick up a lid. The lidapplication tool may then use a gantry or other robotically controlledmechanism to place the lid on the electronic module assembly. In someembodiments, applying a lid on to the seal band comprises applying thelid with a downward force ranging from about 5 pounds to about 30pounds. In some embodiments, the downward force is applied for aduration ranging from about 20 seconds to about 30 seconds. The curingtool 446 heats the electronic module assembly to cure the adhesive usedto form the seal band and lid ties. In some embodiments, the curing toolmay be an oven, and may provide an oven processing temperature rangingfrom about 100 degrees Celsius to about 180 degrees Celsius. Inembodiments, the curing time may range from about 10 minutes to about120 minutes. Other means of curing may also be used to drive the neededchemical reactions for the desired thermal and mechanical propertydevelopment of the adhesives and TIM. These can include using a beltfurnace, microwave curing chamber, or multi-stage thermal curing steps.

FIG. 5 is a flowchart 500 indicating process steps for embodiments ofthe present invention. In process step 550, a chip is placed on alaminate (see 112 of FIG. 1). In process step 552, a seal band is formedon a laminate (see 202 of FIG. 2). In process step 554, lid ties aredeposited (see 208 of FIG. 2). In process step 555, a thermal interfacematerial (TIM) is applied. This may be performed with the same tool (442in FIG. 4) as that which dispenses the seal band and/or lid tiematerial, by using an additional dispensing syringe. The order of someof the steps can vary. The dispensing of the TIM material may be doneeither before the seal band and lid tie dispense or after. In processstep 556, a lid is applied (see 104 of FIG. 1). In process step 558, themodule assembly is cured to stiffen the seal band and lid ties. Notethat while an exemplary order is shown in FIG. 5, the steps may beperformed in a variety of sequences. For example, the lid ties may beapplied before the seal band is formed. In some embodiments, thelocation of the lid ties may be determined by the location of discreteelectrical components (see 328 of FIG. 3).

Embodiments of the present invention provide an improved electronicmodule assembly and method of fabrication. A patterned array of adhesiveis deposited on a laminate, to which a chip is attached. Each region ofadhesive is referred to as a lid tie. A lid is placed on the laminate,and the lid is in direct physical contact with the lid ties. The lidties serve to add stability to the laminate and reduce flexing duringthermal processing and mechanical stress. This improves the coplanarityof the ball grid array, resulting in improved product yield. Anotheradvantage provided by embodiments of the present invention is reducedthermal interface material (TIM) tearing. TIM tearing is caused byflexing of the laminate, and can result in voids in the TIM. These voidsdegrade the ability of heat to transfer from the chip. Therefore, TIMtearing can affect performance and reliability of a chip. By using thelid ties, laminate warpage and flexing is reduced, which in turn reducesthe TIM tearing. Another advantage provided by embodiments of thepresent invention is improved burn-in (BI) resiliency when modules maybe socketed using probes that are pressed against the bottom surfacecontacts to establish electrical connections while the module is heatedand electrically tested. During this BI test, unsupported regions of thelaminate can undergo permanent deformation resulting from the load fromthe probes. Lid ties may serve to minimize this deformation and preventpotential electrical reliability defects while also preserving a planarsurface when needed for subsequent joining operations. Whileillustrative embodiments show attaching of chips to laminates,embodiments of the present invention may also be utilized to attachlaminates to other laminates.

Although the invention has been shown and described with respect to acertain preferred embodiment or embodiments, certain equivalentalterations and modifications will occur to others skilled in the artupon the reading and understanding of this specification and the annexeddrawings. In particular regard to the various functions performed by theabove described components (assemblies, devices, circuits, etc.) theterms (including a reference to a “means”) used to describe suchcomponents are intended to correspond, unless otherwise indicated, toany component which performs the specified function of the describedcomponent (i.e., that is functionally equivalent), even though notstructurally equivalent to the disclosed structure which performs thefunction in the herein illustrated exemplary embodiments of theinvention. In addition, while a particular feature of the invention mayhave been disclosed with respect to only one of several embodiments,such feature may be combined with one or more features of the otherembodiments as may be desired and advantageous for any given orparticular application.

What is claimed is:
 1. A method for fabricating an electronic moduleassembly, comprising: placing a chip on a laminate; forming a seal bandon the laminate; depositing a patterned array of stabilizing lid ties tothe laminate; applying a lid on to the seal band; and curing theelectronic module assembly.
 2. The method of claim 1, further comprisingapplying a thermal interface material layer on the chip.
 3. The methodof claim 1, further comprising applying electric components to thelaminate.
 4. The method of claim 1, wherein depositing a patterned arrayof stabilizing lid ties to the laminate is performed with a syringe. 5.The method of claim 1, wherein depositing a patterned array of lid tiesto the laminate comprises depositing an outer ring of lid ties and aninner ring of lid ties.
 6. The method of claim 1, wherein forming a sealband on the laminate comprises forming a plurality of seal band lidties.
 7. The method of claim 1, wherein curing the electronic moduleassembly comprises heating the electronic module assembly in an ovenwith an air temperature ranging from about 100 degrees Celsius to about180 degrees Celsius.
 8. The method of claim 1, wherein curing theelectronic module assembly comprises heating the electronic moduleassembly for a duration ranging from about 10 minutes to about 120minutes.